1. Field of the Invention
This invention relates to novel aggregate-TiO.sub.2 pigment products comprising in each 100 parts, by weight, at least 50 parts, by weight, particulate titanium dioxide, used alone or in a combination with other pigmentary and subpigmentary raw materials, bound intrinsically with the aid of inorganic and/or organic cements/adhesives.
In a preferred embodiment, this invention relates to aggregate pigment products in which particles of titanium dioxide and other pigmentary, subpigmentary and nonpigmentary components are coflocculated and cemented with the aid of in-situ synthesized calcium-silico-aluminate or similar complex (multicomponent) functional microgels.
2. Discussion of the Relevant Art
White pigments encompass a class of particulate materials which are essentially colorless, insoluble, nontoxic, reasonably nonabrasive, and have dimensions favoring a diffuse reflection, or scattering, of light constituting the visible portion of the electromagnetic spectrum with wavelengths ranging from 420 nm for violet to 660 nm for red.
In accordance with the laws of physical optics, maximum scattering of light occurs when a propagating light wave encounters in its path an obstacle, a pigment particle as the case in point, whose dimensions are equal to one-half of the length of the impinging wave. At equal particle dimensions, pigmentary materials with higher refractive indexes, whose values range from 1.41 for silica to 2.73 for rutile, scatter the light more efficiently than those with lower ones.
The most elementary physical model of light scattering may be considered one in which monochromatic light is diffracted by a single spherical particle. Maximum diffraction of the blue, green and red portions of the light spectrum (additive primary components of light) is obtained with particle diameters of about 150 nm, 200 nm and 250 nm, respectively. By integrating the comprehensive spectral response for a single spherical particle scattering polychromatic light, mapped as a function of particle diameter, one can calculate that the maximum light scattering occurs when this diameter is equal to about 200 nm.
The above physical textbook model of light scattering by a single spherical particle has been promoted for a long time in publications and product bulletins by the most technologically advanced titanium dioxide (TiO.sub.2) pigment manufacturers (e.g., Du Pont's Bulletin H-12565, 12/88, TI-PURE.RTM. Titanium Dioxide for Plastics). Although correct from the standpoint of physical optics, the above model is useless, or even misleading, in application to real-life systems. It should be strongly emphasized that there are ultimately no spherical pigment particles in any TiO.sub.2 -containing end-use formations found in practice that scatter light according to the elementary textbook model discussed previously. Instead, the originally discrete spherical TiO.sub.2 particles occur in the latter formations in the form of complex aggregates (flocs) whose specific shapes and individual contributions to the overall light-scattering efficacy of the integral end-use formations cannot be described with the aid of available mathematical tools and physical models. It is possible, of course, to prepare, with a great deal of effort, artificial miniature formations reasonably amenable to modeling and mathematical treatment in which each single spherical particle of TiO.sub.2 remains discrete; however, the spatial concentrations of pigment particles in such formations would be too low to serve any practical purposes.
It should further be borne in mind that TiO.sub.2 pigments constitute but a fraction of the total tonnage of pigments used in the paper, paint and plastic industries. A useful model of light scattering by pigments must, therefore, be applicable both to any arbitrary pigment shape (virtually all inorganic pigments, other than TiO.sub.2, being nonspherical anyway) as well as integral end-use formations containing these pigments, such as paper-coating and paint films, filled paper or pigmented plastics.
Let us consider, for example, a single, highly aniso-metric particle of kaolin clay in the form of a hexagonal platelet. The light waves of different lengths impinging upon such multifaceted platelets are scattered with different intensities, depending upon how closely the dimensions of a particular facet of this platelet approximate one-half of the length of the impinging light wave. Among the multitude of geometrical facets by which the impinging light wave may be scattered are, for example, platelet faces (in the x,y plane) or edges and protrusions from platelet surfaces (in z direction). Moreover, the impinging light waves are scattered independently by each of the six triangular tips of a hexagonal platelet, the shorter waves being scattered more efficiently closer to the tips, across shorter distances, while the longer waves are being scattered more efficiently farther from the tips, across longer distances. Since the ability to scatter light is a universal property of both particulate and extensive matter, even an "infinitely" large, most precisely polished mirror also scatters light, though only to a very negligible extent. In general, all light waves, regardless of lengths, scatter with different intensities across all physical obstacles encountered in their path, such as individual particles or parts of aggregated matter, grain boundaries or sites of localized stress concentrations giving rise to elasto-optical effects.
It should be emphasized, in the above context, that the term "pigments" (specifically, white pigments) denotes a pragmatic class of particulate materials, useful in the trade, whose features are defined by a convention. In the very minimum, pigments must consist to a predominant extent of particles whose dimensions uniquely favor the scattering of light, not so much with regard to the performance of individual particles but primarily with regard to that of the resultant end-use formations containing these particles. The latter requirement necessitates that pigments additionally possess certain specific features and performance properties, whose scope is not fixed, however, but expands in keeping pace with the scientific and technological advancements in the field of pigments.
Whether a solid particle can be classified as pigmentary depends not only on raw physical dimensions but also on the particle's morphology. Hence, monolithic, spherical, virtually perfectly isometric, single-faceted particles of TiO.sub.2, or organic pigments, cease to be pigmentary for all practical purposes when their particle diameters exceed about 1 .mu.m. On the other hand, multifaceted pigment particles, such as inherently aggregated clusters of elementary, ultrafine (subpigmentary) particles of precipitated silica or metal silicates can be as large as 10 .mu.m or even 20 .mu.m e.s.d. (equivalent spherical diameter) and still be pigment worthy. Regardless of their morphological features, however, all discrete particulate materials with dimensions finer than 0.1 .mu.m e.s.d. are not pigment worthy, being classified as "subpigmentary." It should be pointed out, though, that inherently fine-particle-size pigment products, such as TiO.sub.2 or high-glossing kaolin clay, usually contain substantial proportions of subpigmentary particles.
Typical commercial TiO.sub.2 pigment products consist of spherical particles that are essentially 100% finer than 1.5 .mu.m; 98-99% finer than 1 .mu.m; and 35%-50% finer than 0.3 .mu.m in diameter. Conspicuously, particles with a diameter of 0.2 .mu.m, claimed in the literature to be the most favorable for light scattering, constitute but a minor proportion of the total mass of commercial TiO.sub.2 pigment products. One may raise a logical question, therefore, as to why after so many decades of industrial use there are still no TiO.sub.2 pigment products on the market having essentially all particles of about 0.2 .mu.m in diameter. The above question becomes particularly intriguing in view of the fact that monodisperse fractions of the above type can be obtained with relative ease, as demonstrated repeatedly in laboratory comminution work carried out by the applicant.
The answer to the above question will become clear from the considerations to follow. First of all, a spherical shape for mineral pigment particles is disadvantageous in many respects. Spheres, which are perfectly isometric, single-faceted geometric bodies, scatter the light more selectively, hence, less efficiently, than analogous anisometric particles of an equivalent mass. Furthermore, spheres have an inherent tendency to form dense, closely packed formations (ensembles) with a low void volume, characterized by a low light-scattering efficacy. A closely packed ensemble of a monodisperse population of spheres has a maximum void volume of only about 26%, which can fall below 15%, or even 10%, for analogous polydisperse populations of spheres. The formation of closely packed ensembles of pigment particles, particularly spherical ones, is unavoidable in paper coating and filling, or in paints, causing many potential light-scattering sites to become inaccessible to the impinging light waves. Hence, the integral light scattering of a formation of closely packed spherical particles is invariably much lower than the sum of potential light-scattering effects attainable with the individual component particles scattering light as discrete, optimally separated spheres.
Indeed, decades of industrial experience have shown invariably that the light-scattering efficacy of pigmented formations containing high levels of TiO.sub.2 particles (in a state of "overcrowding") is drastically reduced. As a con-sequence, the use of high proportions of TiO.sub.2 in the overall pigmentation of commercial end-use systems is economically justified only in such special applications for which the opacifying action of low-refractive-index pigments of the prior art is simply too weak, e.g., manufacture of very thin bible paper, coating of kraft board, or formulation of high quality paints and lacquers.
Contrary to predictions of the physical textbook model of light scattering by a single spherical particle, the optical performance of an essentially monodisperse TiO.sub.2 pigment with particles of about 0.2 .mu.m in diameter, prepared from a commercial TiO.sub.2 pigment product with the aid of a novel comminution process, was found to be significantly poorer than that of the starting coarser material. The substandard optical performance of the above monodisperse TiO.sub.2 fraction is explained by excessive flocculation, leading to the formation of very dense, closely packed flocs with a strongly reduced light-scattering efficacy. It should also be borne in mind that TiO.sub.2 pigments are used predominantly in a combination with low-refractive-index co-pigments, which are polydisperse systems with a wide spread of particle dimensions. While the above use of co-pigments is mostly beneficial, oversized particles present in co-pigments have a rather well-documented adverse effect upon the flocculation characteristics of the relatively very small TiO.sub.2 particles.
The effect of larger particles on the flocculation tendency of smaller particles was first described by V.D. Samygin et al. in the article titled "Mechanism of Mutual Flocculation of Particles Differing in Size" (translated from Kolloidnyi Zhurnal, Vol. 30, No. 4, pp. 581-586, July-August, 1968), dealing with flocculation phenomena in flotation processes. According to the above article, the rate of adhesion of fine particles to coarser ones may be higher by a factor of 10.sup.3 -10.sup.4 than the rate of cohesion between finer particles. Applicant's subsequent research work showed that the above phenomenon is universal and is encountered in both wet and dry disperse systems. For example, coarser and more abrasive particle aggregates were obtained through calcining (sintering) very-fine-particle-size clay feeds whose particles range from 0.1 .mu.m to 2.0 .mu.m e.s.d. than by calcining analogous feeds purged from essentially all particles larger than 1.5 .mu.m e.s.d. by centrifugal fractionation.
While the refractive index and light-scattering efficacy of titanium dioxide, particularly futile, are highest among all white pigments, the optical-performance potential of TiO.sub.2 pigments is only fractionally utilized in practical applications. For example, in experiments carried out by the applicant, the light-scattering coefficients of optimally spaced, specially dispersed commercial TiO.sub.2 pigments, measured with the aid of ultrathin films (50-100 mg/m.sup.2) deposited on optically flat black glass plates, were found to approach 3 m.sup.2 /g. The same pigments used in paper filling or other similar applications usually have a light-scattering coefficient of about 0.45-0.5 m.sup.2 /g.
It is the object of the present invention, therefore, to provide a fundamentally novel, if not revolutionary, approach to the manufacture of aggregate pigment products with an "expanded" pigment lattice, prepared from TiO.sub.2 pigments of the prior art used in the state "as is" or additionally processed. The approach in question is considered "revolutionary" inasmuch as it is both diametrically opposite to, as well as explicitly counterindicated by, the practices and doctrines of the prior art, as shall be explained in more detail in the discussions to follow.
Interspacing of high-refractive-index pigment particles with low-refractive-index ones has been a standing practice in the art since the introduction of lithopone pigments in about 1875. The latter pigments are obtained by coprecipitating birefringent zinc sulfide (refractive indices 2,356 and 2,378), used in proportions of from 30-60%, by weight, with barium sulfate (refractive index 1.64). Lithopone is thus the prototype of all composite pigments in which particles of high-refractive-index pigments, such as zinc sulfide (ZnS) or TiO.sub.2, are "extended" (interspaced) with pigment particles of significantly lower refractive indexes, such as barium sulfate or clay.
Indeed, as TiO.sub.2 was introduced on the market in 1919, quickly becoming the dominant high-refractive-index white pigment, it became instantly clear that the most economic performance of the latter is obtained when used in blends with less expensive, low-refractive-index co-pigments, such as barium sulfate. It has also been recognized, however, that a great deal of detrimental selective fractionation and flocculation occurs in practical applications involving the use of such loose pigment blends. Hence, various composite pigment products have been developed in which the "primary" (high-refractive-index) TiO.sub.2 pigment was first intimately blended with, and subsequently affixed to, "secondary" pigments (extenders) having substantially lower refractive indexes to attain a permanent immobilization of all particulate species relative to each other. The above two key processing elements, i.e., maximum homogenization of the particulate component species prior to their immobilization and a subsequent permanent cementing of the resultant heteroaggregates (to ensure their mechanical integrity), are indispensable, though not always sufficient, for a successful synthesis of all composite pigments.
The initial main approaches to the manufacture of TiO.sub.2 -containing composite pigments involved either a precipitation of TiO.sub.2 in a slurry of a secondary (extraneous) pigment, or a simultaneous coprecipitation of both TiO.sub.2 and the secondary pigment followed by dewatering, calcining and pulverization. The above composite pigments, whose manufacture was based in part on a simulation of the lithopone process, were called "coalesced" composite pigments. A thorough intrinsic cementation of particle aggregates of the extended-TiO.sub.2 pigments synthesized in the above-mentioned manner, ensuring these aggregates' mechanical integrity, was obtained by sintering during calcining.
Fundamentally different, novel approaches to the manufacture of composite pigments of the extended-TiO.sub.2 type, based on coflocculation of pigmentary components dispersed in aqueous media, were disclosed by Alessandroni in U.S. Pat. Nos. 2,176,875, 2,176,876 and 2,176,877. In one of these approaches, for example, the coflocculation process was carried out by adding an extraneous flocculant to an aqueous pigment slurry containing both the (primary) high-refractive-index TiO.sub.2 and the (secondary) low-refractive-index extender. In another approach, the coflocculation of the primary and secondary pigments was attained when a separately prepared aqueous slurry of TiO.sub.2, dispersed with one type of dispersant, was blended with a separately prepared extender slurry, dispersed with another type of dispersant, "antipathetic" to the former one. In both of the above approaches, the flocculated media were filtered, dried and pulverized without employing the calcining step.
Based on the present colloid-chemical experience it is virtually certain, however, that a high degree of detrimental separation and selective aggregation of different particulate species could not have been avoided with the aid of the slow and inefficient flocculation mechanisms employed by Alessandroni. Furthermore, Alessandroni's approaches are devoid of any conceivable adhesion mechanism capable of imparting adequate mechanical integrity to the resultant composite pigments.
U.S. Pat. No. 3,453,131 to Fadner discloses a method for making composite pigments, both white and colored, consisting of functional colloidal particles of ". . . carbon black, acetylene black, iron oxide, Mannox blue, azobisisobutyronitrile, zinc oxide, methyl zimate, sulfur, titanium dioxide, polystyrene, or antimony oxide or mixtures thereof" with diameters ranging from 0.01 .mu.m to 1.0 .mu.m, attached, by means of a "coupling agent," to platy clay particles, ranging from 0.5 .mu.m to 3.0 .mu.m in diameter, used as a carrier medium. The above composite pigments were synthesized by adding 0.5% to 25%, by weight, of an aliphatic acid (coupling agent) into an aqueous slurry of pigmentary components and ". . . mixing the composite suspension for a sufficient time to form the composite colloidal particles."
The resultant "composite particle suspensions" were considered as the final products intended for use in various commercial formulations in which the individual component materials have traditionally been employed in a loose (non-aggregated) state. U.S. Pat. No. 3,453,131 to Fadner also teaches that, "Alternately, the composite particles can be separated from the aqueous medium, for instance, by freeze-drying or by spray-drying, and utilized subsequently in formulating aqueous, non-aqueous or non-liquid composition."
Similarly to Alessandroni, Fadnet does not provide any information with regard to the mechanical integrity of the resultant dried composite particles. An analysis of the functional aspects of Fadner's composite pigment systems, however, clearly points to the lack of any practically significant adhesion mechanism capable of providing such an integrity to the composite pigments in question.
Yet another approach to the synthesis of composite pigments of the extended-TiO.sub.2 type, in which coarse delaminated or calcined clays were employed as the extenders, was disclosed in U.S. Pat. No. 3,726,700 to Wildt. The latter approach relies on forming in situ (in the composite pigment furnish) alumino-silicate or similar gels (of the type used routinely in the TiO.sub.2 -pigment industry for applying surface coatings to TiO.sub.2 particles) instead of on an intentional flocculation of pigment furnishes employed by Alessandroni and Fadnet. The mechanical integrity of Wildt's composite-pigment aggregates is provided by a thermal curing of the in-situ-formed gels, called in the above patent ". . . hydrous oxide of aluminum, silicon, titanium, and mixtures thereof."
In analyzing the colloidal and kinetic aspects of the approach used by Wildt, it is readily understood by those skilled in the art that a detrimental fractionation and selective flocculation of the pigmentary components employed, both according to species as well as size, could not have been prevented during the course of the lengthy synthesis process in which just a single step of digestion takes from 30 to 60 minutes. Furthermore, the above fractionation and selective flocculation were undoubtedly facilitated even more through the use of the dispersion-destabilizing alum. Although a permanent immobilization of TiO.sub.2 particles relative to the extender particles was undoubtedly achieved in Wildt's composite pigments, there also is virtually no doubt that the latter immobilization was realized through an attachment of "blobs" of badly flocculated TiO.sub.2 particles to the coarse extender particles.
The two most fundamental objections to be raised with regard to Wildt's composite pigments are (a) using far too few extender particles (calculated to be present in the system according to the relative proportions of TiO.sub.2 and coarse-particle-size extenders employed) to interspace effectively the available TiO.sub.2 particles; and (b) a total mismatch between the dimensions of the excessively coarse "spacer" (extender) particles in relation to the very much smaller TiO.sub.2 particles to be "interspaced." Furthermore, since Wildt does not provide any data pertaining to the light-scattering efficacy of his composite pigments, e.g., in a head-to-head comparison with the rutile pigment used as the raw material, it is virtually impossible to draw unambiguous conclusions as to the true source of the improvement of the hiding efficacy of paint systems formulated with the aid of the composite pigments in question. As is well known to those skilled in the art, however, the hiding efficacy of TiO.sub.2 -based paints can also be increased by blending into the latter loose (extraneous) particulate high-oil-absorption silicate materials of the same type as the gel synthesized in situ in Wildt's composite pigments.
The principal concept of a permanent interspacing of high-refractive-index pigment particles with low-refractive-index extender particles, to prevent a detrimental crowding of the former, has been at the foundation of the design and manufacture of all composite pigments of this type known in the prior art. The above doctrine of interspacing is formulated in most explicit terms in U.S. Pat. No. 3,726,700 to Wildt, who states in col. 1, lines 45-56: "The optimum spacing to give the greatest efficiency of light scattering per TiO.sub.2 particle is generally considered to be one half the wave length of light, or 0.20-0.25 microns. However, in the interest of obtaining higher total opacity of the system, it may be necessary to sacrifice light-scattering efficiency by closer spacing than optimum by addition of more TiO.sub.2. At a TiO.sub.2 volume concentration of about 30% (approx. 40% by weight--conversion from volume to weight added by the applicant), further additions no longer increase the total opacity because of a rapid rate of decrease of opacity with increased crowding."
In analyzing the reasons for the apparent lack of success in attaining the goal of a statistically uniform interspacing of TiO.sub.2 particles with extender particles attempted in the prior art, attention must be drawn to yet another key processing element indispensable to the successful manufacture of the composite pigments in question. This key processing element is, in addition to the already discussed homogenization and cementing, an instantaneous, for all practical purposes, coflocculation (immobilization) of any and all heterodisperse and polydisperse particulate raw materials used in synthesizing composite pigments. As is well known to those skilled in the art, however, a viable method for an instantaneous flocculation (immobilization) of disperse particulates, preventing their separation and selective aggregation, had not been known in the prior art before it was disclosed in U.S. Pat. No. 5,116,418, to Kaliski ("Process for Making Structural Aggregate Pigments," as well as in the co-pending patent application Ser. No. 07/775,025 ("Functional Complex Microgels with Rapid Formation Kinetics") filed Oct. 11, 1991, now abandoned; Ser. No. 07/811,603 ("TiO.sub.2 -Containing Composite Pigment Products") filed Dec. 23, 1991, now U.S. Pat. No. 5,312,484; and Ser. No. 07/811,623 ("Low-Refractive-Index Aggregate Pigment Products") filed Dec. 23, 1991, now U.S. Pat. No. 5,279,663; the above-mentioned patents as well as applications being incorporated herein by reference. Moreover, extender pigments, suitable for attaining a geometrically uniform interspacing of populations of TiO.sub.2 particles typical of present commercial TiO.sub.2 pigment products, would have to have particle diameters ranging from about 0.05 to 0.1 .mu.m. It should be pointed out, though, that ultrafine (subpigmentary) particulate materials of the above type were never available on the market and, if synthesized, would be almost impossible to disperse, store and utilize in typical industrial practices of the prior art.
Novel approaches toward interspacing of TiO2-pigment particles with particles of specially treated commercial extender pigments, to synthesize aggregate composite pigment products of the extended-TiO.sub.2 type, were disclosed in the previously mentioned co-pending patent application Ser. No. 07/811,603. New types of aggregate-TiO.sub.2 pigment products (fundamentally different from the extended-TiO.sub.2 composite pigment products of the prior art), in which arbitrary levels of TiO.sub.2 -pigment-lattice expansion are obtained using, among other things, in-situ-synthesized subpigmentary particles, will be disclosed hereinafter.
The issue of an optimized extension of TiO.sub.2 pigments, treated extensively in the literature in the past several decades, was most fittingly summarized by J.H. Brown in the article titled "Crowding and Spacing of Titanium Dioxide Pigments," issued in the Journal of Coating Technology, Vol. 60, No. 758, Pages 67-71, March 1988, dealing with hiding properties of nonporous paints. In the above article Brown dismisses the usefulness of particulate extenders, opting instead for coatings deposited on the surface of TiO.sub.2 particles. His general conclusions are as follows: "For geometric reasons, the maximum size extender particles intended to improve hiding is limited by rutile size and volume concentration. Maximum size of hiding effective extender is small, less than commercial products and dispersion processes can deliver. Hiding improvements can, however, be obtained through uniform spacing of futile by coatings on rutile particles. The following requirements should be met: (1) Composition--"Extender"/rutile combinations should be less than 40 vol % extender/60 vol % futile; (2) Configuration--Extender should be present as a coating of less than 0.05 .mu.m thickness on rutile; and (3) Application--The concept is applicable to paints of pigment volume concentration greater than 20%. The hiding power advantage of such a product over conventional rutile could be as much as 10%."
The inescapable conclusion drawn from an historical assessment of the above subject matter is that the approaches taken in the prior art with regard to the improvement of the optical-performance efficacy of TiO.sub.2 pigments were grossly misguided. First of all, the phenomena of interspacing (extension) of TiO.sub.2 particles were, as a rule, treated in an unrealistically isolated manner, without giving due consideration to such important phenomena as flocculation and co-flocculation occurring in the complex systems encountered in practice on the one hand and the polydisperse aspects of commercial TiO.sub.2 products on the other. Secondly, the potential beneficial effects of a correct interspacing and extension of TiO.sub.2 were badly underestimated in the prior art, as demonstrated by the data disclosed in the previously mentioned co-pending patent application Ser. No. 07/811,603, and as further demonstrated hereinafter.
It is worth emphasizing that while all composite pigments are, de facto, aggregates, the aggregation as such, specifically, a controlled aggregation, has never been employed in the prior art as an independent vehicle for the improvement of the optical properties of composite pigments. All such improvements have always been attempted through an interspacing of particles of the primary (high-refractive-index) pigments with particles of secondary (low-refractive-index) pigments/extenders. The reason for this becomes clear considering that the most detrimental side effects encountered in practical applications of TiO.sub.2 pigments, used alone or in blends with other pigments, are associated largely with undesirable flocculation phenomena, particularly those of a selective type.
The potential for improving the light-scattering properties of entire pigment populations by aggregating in situ pigment fines (subpigmentary fractions present to some extent in virtually all commercial pigment products), whose dimensions in a discrete state are too small for efficient light scattering, was first discovered by the applicant and published in the Journal of the Technical Association of the Pulp and Paper Industry (TAPPI), Vol. 53, No. 11, November 1970, Pages 2077-2084 ("Performance of Some Commercial Clays in Starch-Containing Paper-Coating Films; Part I. Black Glass Plates as Model Substrates"), preceded by a presentation at the TAPPI Coating Conference held in Houston, Texas, May 3-4, 1970. The above publication contains, among other things, a graphical presentation (FIGS. 6 and 7) of the light-scattering coefficients (at the wavelengths of 457 and 577 nm) of three different clay pigments made into starch-containing coating colors and deposited as films on optically flat black glass plates as coating substrates and assessed as a function of the binder-volume fraction in the coating. The slopes of the curves representing the light-scattering coefficients of No. 1 and No. 2 coating clays ascend initially with the increasing binder-volume fractions and, after reaching the maximum values at a binder-volume fraction corresponding to about 5 parts of starch per 100 parts of clay, by weight, descend as the binder level is further increased.
This initial increase of the light-scattering coefficients is explained in the above publication ". . . by an aggregation of clay fines effected by the initial addition of binder. The aggregates of ultrafine particles, which are understood here as assemblies of a very few such particles, should scatter the light more effectively than the individual components." The subsequent steady decline of the magnitude of the light-scattering coefficients is explained as follows: "An increase of the binder content of the coating systems beyond the F.sub.bv (binder-volume fraction--explanation added by the applicant) value of 0.080 (5 parts starch per 100 parts clay, by weight) appears to cause a further growth of the assemblies of pigment particles, so that the optimum dimensions of the light-scattering sites are exceeded."
With the relatively coarse mechanically delaminated clay, having only minor proportions of pigment fines (particles smaller than 0.1-0.2 .mu.m e.s.d.), the light-scattering coefficients of the coatings declined from the very first incremental addition of the binder because of the scarcity of ultrafine particles amenable to a beneficial aggregation. The intrinsically coarser structure of the coating films containing the mechanically delaminated clay, compared to the structure of coatings containing No. 1 and No. 2 clays, was verified with the aid of a new empirical parameter called "Rho" (after the Greek letter .rho.), defined in the above publication as the ratio of the numerical values of the light-scattering coefficients determined at 577 nm and 457 nm for the same coating film. With coating films characterized by intrinsically finer structures, such as binderless coatings or coatings with a low binder-volume fraction, the magnitudes of the corresponding "Rho" parameters are low. As the intrinsic coating structure becomes coarser, as was the case with all coatings discussed in the above publication in which the binder content was continuously increased, the magnitude of Rho increases accordingly. Below a certain specific binder-volume fraction (corresponding to about 5-8%, by weight), the coarsening of the coating structure is beneficial; hence, an increasing Rho value is associated with an increase of the light-scattering coefficients. Above this specific binder-volume fraction, however, the coarsening of the coating structure becomes excessive, the increasing Rho values being associated with a decrease of the light-scattering coeficients.
As is evident from the above considerations, further verified by ample practical experience, the light-scattering efficacy of both white pigments and end-use formations containing such pigments can be significantly improved by a purposeful in-situ aggregation of pigment fines. It is also obvious, from the standpoint of physical optics, that the beneficial in-situ aggregation of pigment fines applies universally to all white pigments, regardless of whether they are made of low- or high-refractive-index materials. The above-mentioned publication by Kaliski (TAPPI Journal, Vol. No. 11, November 1970, Pages 2077-2084) thus established the scientific foundations for an entirely new pigment technology opening the way to designing and manufacturing new lines of pigment products with an improved optical performance, such pigment products being synthesized by a controlled aggregation of commercial and/or novel pigmentary raw materials, used alone or combined with various subpigmentary and/or nonpigmentary particulates.
Indeed, the first patent pertaining to the manufacture of aggregate pigments with an improved optical performance (U.S. Pat. No. 4,075,030: High Bulking Clay Pigments and Methods for Making the Same) was issued in 1978 to Bundy et al., followed by related patents by other inventors. It should be emphasized, however, that none of the patented aggregate pigments was synthesized under conditions allowing a satisfactory control of the aggregation process, attainable only with the aid of an instantaneous, indiscriminate and complete flocculation. A flocculation process of the above-mentioned type was unknown in the prior art, however, before the previously-mentioned disclosure by the applicant. Moreover, as documented amply by industrial experience, the problem of imparting an adequate mechanical integrity to aggregate pigments, while simultaneously generating controlled (beneficial) intrinsic aggregate structures, has never been resolved satisfactorily in the technology of aggregate-pigment products of the prior art.
Novel methods for the manufacture of practically countless types of structural aggregate pigments with exotic compositions, enhanced optical properties, excellent mechanical integrity, and unique functional properties, based, among other things, on the beneficial in-situ aggregation of pigment fines, were disclosed in the previously mentioned U.S. Pat. No. 5,116,418 to Kaliski and U.S. Pat. Nos. 5,279,633 and 5,312,484.
In accordance with the foregoing and disclosures to follow, it is an object of the present invention to provide compositions for novel aggregate-TiO.sub.2 pigment products consisting predominantly of TiO.sub.2 as the raw material and synthesized by the general method disclosed in the above mentioned U.S. Pat. No. 5,116,418 to Kaliski, and by any other methods and approaches known in the prior art applicable, in principle, to synthesizing the aggregate-pigment products under discussion.
It is also an object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products comprising in each 100 parts, by weight, at least 50, preferably more than 77, parts, by weight, of intrinsically cemented particulate TiO.sub.2 derived from prior-art TiO.sub.2 pigment products in the state "as is," or comminuted further, beyond the limits of comminution practiced in the prior art, to particle dimensions essentially 100% finer, by weight, than from 0.3 .mu.m to 0.9 .mu.m in diameter; borderline pigmentary, with particles essentially 100% finer, by weight, than 0.2 .mu.m in diameter; or even subpigmentary, with particles essentially 100% finer, by weight, than 0.1 .mu.m in diameter.
It is a further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products (consisting predominantly of TiO.sub.2 as the raw material) made by a controlled in-situ aggregation of TiO.sub.2 fines to render the latter more effective with regard to light scattering.
It is a yet further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products in which the statistical pigment lattice formed by aggregated particles of TiO.sub.2 is expanded at will with the aid of one or more of the following materials:
(a) in-situ (in the starting pigment furnish) synthesized inorganic, organic and/or (hybrid) inorganic/organic cements/adhesives; PA1 (b) in-situ-synthesized mineral subpigmentary particulates; PA1 (c) mineral subpigmentary particulates introduced into the system (pigment furnish) from an extraneous source; and PA1 (d) extraneous particulate and/or water-soluble non-pigmentary materials. PA1 (a) in-situ-synthesized subpigmentary mineral particulates, up to 25 parts, by weight; PA1 (b) extraneous, mechanically deagglomerated/comminuted subpigmentary mineral particulates, up to 25 parts, by weight; PA1 (c) inorganic, low-refractive-index pigmentary diluents, up to 45 parts, by weight; PA1 (d) organic, low-refractive-index, non-film-forming pigmentary diluents, up to 10 parts, by weight; PA1 (e) color dyes, up to 5 parts, by weight; PA1 (f) carbon black, up to 0.25 parts, by weight; and PA1 (g) organic, cationically active chemical compounds with at least two reactive groups in each molecule, up to 0.5 parts, by weight; PA1 (a) in-situ-synthesized inorganic microgel cements obtained by cross-linking of sodium-silico-aluminate and similar transient, chemically reactive subcolloidal hydrosols with bivalent and/or multivalent inorganic salts; PA1 (b) in-situ-synthesized inorganic cements obtained by a hydrolysis of metal chlorides, with the aid of ammonia, in said TiO.sub.2 and other pigmentary and/or subpigmentary, essentially dry, raw materials (having a moisture content of from 0.4 to 1%, by weight) intimately blended with in-situ-synthesized and/or mechanically deagglomerated/comminuted subpigmentary cement precursors; PA1 (c) in-situ-synthesized polysalts obtained by a reaction between organic (monomeric or polymeric) dispersants and organic cationic polyelectrolytes; PA1 (d) in-situ-synthesized, predominantly inorganic (hybrid, inorganic/organic) microgel cements obtained by cross-linking of sodium-silico-aluminate and similar transient, chemically reactive subcolloidal hydrosols with a blend of bivalent (and/or multivalent) inorganic salts and organic, cationically active chemical compounds with at least two reactive groups in each molecule; PA1 (e) in-situ-synthesized complexes, which can be predominantly inorganic or predominantly organic, obtained by a reaction between organic, cationic polyelectrolytes and inorganic, anionic dispersants, such as alkali-metal phosphates or alkali-metal silicates; and PA1 (f) extraneous organic cements/adhesives selected from the group consisting of the following materials: PA1 (a) up to 20 parts, by weight, in-situ-synthesized inorganic or predominantly inorganic (hybrid, inorganic/organic) complex microgel cements, regardless of whether employed as the only cements/adhesives in the system or in a combination with other organic, in-situ-synthesized and/or extraneous cements/adhesives; PA1 (b) up to 20 parts, by weight, in-situ synthesized inorganic cements obtained by hydrolyzing metal chlorides, with the aid of ammonia, in the presence of subpigmentary (in-situ-synthesized and/or mechanically deagglomerated/comminuted) cement precursors intimately blended with said TiO.sub.2 and other pigmentary and/or subpigmentary raw materials, regardless of whether employed as the only cements/adhesives in the system or in a combination with other organic, in-situ-synthesized and/or extraneous cements/adhesives; PA1 (c) up to 10 parts, by weight, extraneous, organic cements/adhesives, active basis, when employed in addition to the following: PA1 (d) up to 10 parts, by weight, cements/adhesives, active basis, when employed as the only cements/adhesives in the system, selected from the group consisting of the following materials: PA1 (e) up to 15, parts, by weight, ultrafine dispersions of thermoplastic adhesives (with particles essentially 100%, by weight, finer than 0.1-0.2 .mu.m), active basis, when used as the only cements/adhesives in the system.
It is a still further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products additionally containing minute proportions of color dyes to obtain color-neutral end products devoid of the inherent yellow hue of commercial TiO.sub.2.
It is a yet further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products additionally containing up to 5 parts, by weight, of color dyes to render the resultant products directly applicable to the coloring of paper, paints, plastics and synthetic fibers.
It is a yet further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products additionally containing specially deagglomerated carbon black to impart extra-high opacifying properties to these products and thus render them particularly suitable for such applications as the manufacture of lightweight newsprint or paints and lacquers with ultrahigh hiding properties.
A still further object of the invention is to provide compositions for novel aggregate-TiO.sub.2 pigment products additionally containing minute proportions of chemically built-in organic, cationically active compounds with at least two reactive groups in each molecule to impart arbitrary levels of oleophilic properties to these products and thus render them uniquely compatible with, and dispersible in, organic media such as plastics, synthetic fibers and solvent-based lacquers and paints.
A yet further object of the invention is to provide compositions for novel aggregate-TiO.sub.2 pigment products additionally containing extraneously prepared, low-refractive-index inorganic and/or organic pigments, used as diluents, to increase the economy of use of the resultant aggregate pigment products.
It is a further object of the invention to provide compositions for novel aggregate-TiO.sub.2 pigment products in which the particulate ingredients are coflocculated in a controlled manner into pigmentary aggregates whose intrinsic structure and spatial distribution of light-scattering sites provide substantially better light-scattering efficacy, functional properties and economy of use than can be obtained with unadulterated (nonaggregated) TiO.sub.2 pigment products of the prior art.
It is also a particularly special object of the invention to provide principles of qualitative and quantitative formulating of the component raw materials employed, as well as principles of designing optically favorable, intrinsic spatial and structural particulate configurations to arrive at novel aggregate-TiO.sub.2 pigment products whose superior optical performance constitutes a proof that the theoretical models and doctrines accepted in the prior art, and actively supported by the most technologically advanced manufacturers of TiO.sub.2 pigment products, are essentially inapplicable to the end-use systems encountered in commercial practice.